Overall: Project Summary Of the most prevalent cancers in the United States over the last three decades, bladder cancer (BC) has consistently ranked the fifth in annual incidence and the first in medical expenses on a per-case basis. Despite its high prevalence and staggering medical costs, BC is until recently among the least studied cancers with limited efforts in basic, translational and clinical research. Consequently, few assays and biomarkers exist to reliably diagnose BC and predict its progression and few new therapeutic approaches are available to effectively manage BC. Compounding this is the fact that BC is highly heterogeneous comprised of major phenotypic variants and molecular subtypes, as evidenced by previous genetic studies and recent whole- genome/-exome/transcriptome analyses. Over the last five years, the investigators of our PO1 Team have worked together in a highly synergistic and collaborative manner in tackling the biological bases of BC heterogeneity and made significant strides in: discerning the combinatorial genetic drivers of major BC variants; establishing DNA damage and repair as distinguishing features of different types of BC; establishing XIAP as a critical driver of BC invasion; and identifying urothelial subpopulations as potential progenitor cells of BC variants. This renewal application builds on this forward momentum and focuses on a much deeper and expanded scientific theme ? the genetic, carcinogenomic, molecular and cellular underpinnings of BC heterogeneity, by asking several important new questions. What dictates the origin(s) of major BC variants: divergent genetic drivers versus divergent progenitor cells (Project 1; P1)? Do the nitrosamines converted from nicotine in E-cigarette vapor within host cells cause muscle-invasive BC and how do they affect urothelial DNA damage and repair (P2)? And is there a master switch that drives the formation of basal-subtype muscle- invasive BC (P3)? Results from this highly coordinated, collaborative team effort should contribute to the major leap forward in our understanding of the biological bases of BC heterogeneity, leading to the development of novel biomarkers and therapeutics for this highly prevalent but extremely under-studied disease.